1. Field of the Invention
The present invention relates to a record device of the thermal deformation type and, more particularly, to such a device that has improved optical properties.
2. Description Relative to the Prior Art
It is well known to record information in the form of deformations, or ripples, in a thermoplastic film by forming a charge pattern on the film surface corresponding to the information to be recorded, and then heating the thermoplastic film to its melting point. Electrostatic forces produce a deformation pattern in the fluid thermoplastic film corresponding to the charge pattern, the pattern being rendered stable by cooling the thermoplastic film below its melting point. Recently, it has been found that a highly concentrated laser beam can be used to directly form a deformation pattern in a thermoplastic recording material without the use of an electrical charge pattern. For example, U.S. Pat. No. 3,475,760 discloses the direct recording of pictorial information, such as pictures or printed matter, at reduced size on a recording material. In accordance with that disclosure, a scanning laser beam is amplitude-modulated as a function of the pictorial information to be recorded. The recording material is a thermoplastic material that includes a dye strongly absorbant to radiation at the wavelength of the laser beam. As the modulated laser beam scans across the thermoplastic recording material, the material is selectively melted and some of the material is displaced from the exposed area so as to form ridges along the edges of the area. Since the thermoplastic material quickly cools as the laser beam moves to other areas of the material, the deformation pattern is frozen into the recording material. The recorded image can be viewed by well known Schlieren optical techniques.
Even more recently, it has been discovered that thermal deformation recording is suitable for recording information at a high packing density (such as encountered, for example, in video and data processing applications) if the thermal recording material is properly selected. U.S. patent application Ser. No. 862,069, filed Dec. 19, 1977, in the names of H. T. Thomas and J. H. Wrobel, the entire disclosure of which is hereby incorporated herein by reference, discloses several dye-binder combinations useful as recording materials for the high quality, thermal deformation recording of such information. According to that disclosure, a scanning laser beam is pulsed on and off in accordance with the information to be recorded and is used to thermally deform a recording material. The resulting deformations are crater-like in configuration and are generally on the order of one micron in length. Upon illumination, the information is readable through playback optics.
The illumination for playing back a thermal deformation recording of the type disclosed in U.S. Pat. No. 3,475,760 or U.S. patent application Ser. No. 862,069 (such recording hereinafter referred to as "direct thermal recording") can be provided in one of two ways:
(1) The recording laser beam can be used to read the recorded information by reducing its power to a level such that the recording material is not re-melted during the reading process. PA1 (2) A second laser beam having a wavelength at which the recording material is transparent can be used at a relatively high power level for playback, there being no danger of re-melting the recording material since it does not significantly absorb that laser radiation.
Each of these methods has its drawbacks. Reading with the recording laser at a reduced power level results in a playback signal having a lower signal-to-noise ratio (SNR), especially if certain precautions are not taken in designing the readout system (such as using a relatively expensive photodetector suitable for low light detection). For many applications, such as home use, the cost of a high quality, low light detector can be prohibitive. On the other hand, reading the recorded information with a high power laser beam having a wavelength at which the recording material is transparent is not without its own problems, i.e., such a technique requires separate laser systems for recording and playback.